There has been available an e-commerce system to access an online shopping site that constructs a virtual mall with a plurality of virtual shops on the Internet in order to purchase a product (or a service) at a shop that corresponds to each of the virtual shops. In such an e-commerce system, there is a case where, after searching product information based on a keyword specified by a user, the system sorts the products listed in the search result by using attribute information of the products as a sort key. For example, after a product group (sort target data group) is specified by search or product categories, the products are rearranged in an order of one of the conditions, such as “lowest price”, “highest price”, “newly-arrived”, “number of reviews”, and “date of shipment” (see Patent Literature 1 below).